Within the scope of the development of electric or hybrid vehicles, there is a trend towards, and the necessity to, operate high-power batteries in motor vehicles. For optimal utilization, the batteries require cooling to reach a defined operating temperature.
It is well known that batteries, and more specifically, high-performance batteries, have an optimum operating temperature and require as homogeneous a temperature distribution as possible over all cells. Therefore, the technical solutions of prior art aim at directly cooling the motor vehicle batteries by using a refrigerant evaporator by refrigerant direct cooling or contact cooling, respectively. The refrigerant evaporator is integrated into the motor vehicle's refrigeration plant for the air conditioning of the passenger compartment.
DE 10 2009 029 629 A1 discloses a heat exchanger for the tempering of vehicle batteries. The heat exchanger at the same time is established as a holder of the battery units in motor vehicles. In this instance, the surface temperature of the cooling surface is maintained at a level as homogeneous as possible by a defined guiding of the refrigerant through the channels of the multipart tubes.
Further, DE 10 2008 035 400 A1 discloses a device for cooling a heat source of a motor vehicle. A cooling body has several forward flows and several return flows, and a homogenization of the surface temperature of the cooling sheets is obtained by guiding the adjacent flows in forward and backward directions.
Evaporators for the cooling of batteries, also referred to as battery coolers or contact coolers, are integrated into the refrigeration circuit, according to prior art, parallel to the passenger compartment evaporator, or air cooler. Thus, the pressure level at the exit of the battery cooler is equal to the pressure level at the exit of the passenger compartment evaporator. Therefore, the temperature level of the battery cooler is equal to the temperature level of the passenger compartment evaporator. The refrigerant mass flow in the battery cooler, just like in the passenger compartment evaporator, is controlled by a thermostatic expansion valve. The thermostatic expansion valve controls the mass flow in such a way that a defined overheating at the exit of the air cooler and the battery cooler is adjusted.
The prior art has several disadvantages. First, dividing the refrigerant mass flow supplied to the evaporators even for a symmetrical design leads to a slight unevenness in the mass distribution due to minor differences in the flow resistances of the individual flow paths. The refrigerant in the flow path with a lower mass flow proportion will be overheated a little more than the refrigerant in the other flow path. However, due to overheating, the flow resistance in this flow path will increase. This effect is compounded by the control of a defined overheating at the exit of the evaporator after mixing of the partial mass flows.
Second, normally at the exit of the evaporators there is a diphase mixture with a low vapor proportion, but the vapor proportion is not evenly distributed over the flow cross-section. If a division into several flow paths is made, slight differences in the vapor proportion of the single partial mass flows will result. The flow path with the lower vapor proportion will be overheated a little more than the other flow path. As mentioned above, this effect will be compounded by the higher flow resistance of this partial mass flow.
Another disadvantageous effect if the battery and vehicle coolers are operated in parallel is that a clearly higher refrigeration power is required when the drive batteries are dynamically cooled compared to the stationary cooling load due to the electric losses. The dynamic refrigeration power can be higher than double the stationary cooling load. The high refrigeration power in case of dynamic battery cooling causes an undesired high temperature spread in the battery cells due to the relatively bad heat conduction in the cells. The high temperature spread has a negative effect on the life of the battery cells.
Further, the overheating at the exit of the battery cooler causes the refrigerant temperature to increase, hence, an uneven temperature distribution at the surface of the battery cooler. This unevenness in the temperature distribution, due to bad heat conduction in the battery cells, reappears in the cells.
It would be desirable to provide a refrigeration plant and process that realizes an improvement of the temperature distribution in the battery cells.